One’s active in study of rationality (8)
I believe the answer is:
reasoned
'one's active in study of rationality' is the definition.
I can't judge whether this defines the answer.
'one's active in study' is the wordplay.
'active' is an anagram indicator (the letters are active/moving).
'in' is an insertion indicator.
'study' becomes 'read' ('read' can be a synonym of 'study').
'ones' with letters rearranged gives 'sone'.
'sone' placed into 'read' is 'REASONED'.
Can you help me to learn more?
(Other definitions for reasoned that I've seen before include "Deduced or concluded" , "made a logical case" , "Argued logically" , "Spoke logically, thought and drew conclusions" , "Logically derived" .)